Display devices such as cathode ray tubes (CRTs), liquid crystal display devices, vacuum fluorescent displays (VFDs), light emitting diodes (LEDs), and electroluminescence (EL) display devices have been widely used. In recent years, along with significant progress in information technologies, etc., there are rapidly increasing demands in terms of high performance and high functions of display devices; for example, high levels of display quality (high intensity, high definition, large area, wide viewing angle, etc.), low power consumption, space saving, and weight reduction. In response to such demands in terms of performance, sophistication of widely available display devices is actively being made. In addition, plasma display panels (PDPs) have become commercially available in recent years. Further, the research and development of future promising display devices such as field emission displays (FEDs) and flat-panel type vacuum fluorescent displays (fluorescent display tubes) have been actively conducted.
Meanwhile, electron beam or ultraviolet irradiation can cause some zinc oxides (ZnO) to emit blue-green light (i.e. green light containing a blue element). Fluorescent powders of such zinc oxides are characterized by exhibiting high light emission efficiencies upon low-speed electron beam irradiation at an accelerating voltage of 1 kV or less. Therefore, they have been used in practice for vacuum fluorescent displays. However, fluorescent powders have some drawbacks when applied to novel display devices such as field emission displays and flat-panel type vacuum fluorescent displays.
Zinc oxides (ZnO) have been gaining attention in terms of fluorescence compared with other sulfide-based phosphorus materials. Zinc oxides (ZnO) have fluorescence properties that are influenced by structural formations thereof. This is because surface defects of zinc oxides (ZnO) significantly influence the fluorescence excitation bands and intensities.
It has been attempted to form thin-film zinc oxide fluorescent substances by use of electron beam deposition methods, sputtering methods, pulse laser deposition methods, spray pyrolysis methods, and other methods. However, it is still impossible to obtain a practically available thin-film fluorescent substance that exhibits high intensity and high light emission efficiency. In addition, when using the above conventional methods, it is necessary to heat a substrate at a high temperature or carry out heat treatment at a high temperature. Therefore, it is difficult to cause formation of a zinc oxide fluorescent substance thin film on a transparent conductive layer or a color filter without inducing deterioration of such layer or filter. As a result, light emission intensity and color purity deteriorate, which is problematic.
As an aside, JP Patent Publication (Kokai) No. 2005-039131 A discloses a process for producing a zinc oxide single crystal with the use of a hydrothermal synthesis method. Specifically, the document discloses a process for producing a zinc oxide single crystal wafer, which comprises a two-stage heat treatment step involving a pre-heat treatment for realization of low resistance and a subsequent heat treatment for planarization.